Since the advent of Linear Low Density Polyethylene (LLDPE), these linear polymers have benefited their users with physical and mechanical properties suitable for applications such as molded parts, foams, and films.
All conventional linear polymers have drawbacks that in melt processing, especially for film production, can hamper their utilization. These drawbacks are: (1) general lack of melt strength, which may manifest itself in bubble instability during film blowing process or in surging and edge weave in cast films; and (2) a low or flat response of viscosity to shear; this latter may manifest itself in higher melt viscosity leading to undesirable higher motor loads and torque requirements for extruders.
To mitigate these drawbacks, processors have for decades, resorted to blending in branched polymers such as conventional, high-pressure process produced low density polyethylene (LDPE), into the LLDPE. However, such post-reactor additions can add to the cost as well as diminish some of the very desirable physical property improvements associated with LLDPE such as its toughness properties.
There is a commercial need therefore for a LLDPE that will have improved melt strength and improved shear thinning in response to applied shear while maintaining the important physical and mechanical properties.
We have discovered that a substantially non-blended LLDPE having relatively narrow molecular weight distribution, can have high melt strength, generally conferring good bubble stability during the melt processing of these polyethylenes in blown film processes, high melt index (MI), and high MIR, generally indicative of a relatively high degree of shear thinning, both parameters indicating directionally improved processability.
This substantially non-blended LLDPE may be made by a gas phase polymerization of ethylene and an α-olefin. The polymerization may be catalyzed by a supported metallocene catalyst combination. The supported catalyst will include metallocenes according to the following structures: The indenyl or the tetrahydroindenyl (THI) rings may be substituted. The substituents may be linear or branched alkyl groups one to twenty carbons long, siloxy and its derivatives, phenyl and its derivatives, or any substituents group that gives an active catalyst system.
The two metallocene components may be placed on separate supports and separately injected into the reactor or, alternately, they may be “co-fed” (that is, the two separately supported metallocene components are introduced into the reactor(s) as a mixture or blend) or the two metallocene components may be “co-deposited” (that is, formed by each being placed on the same support). The support materials are inorganic silicas and inorganic oxide materials, which include those of Groups 2, 3, 4, 5, 13 or 14 metal oxides. In another embodiment, the catalyst support materials include silica, alumina, silica-alumina, and mixtures thereof. The carrier of the catalyst of this invention has a surface area in the range of from 10-700 m2/g, pore volume in the range of from 0.1-4.0 cc/g and average particle size in the range of from 10-500 μm. Other embodiments of our invention include the ratio of the two metallocenes a:b in the range of from 90:10-10:90 in a polymerization process. The catalyst system may be activated by any means known by those of skill in the art, such as by utilizing methylalumoxanes, aluminum alkyls, and mixtures thereof, non-coordinating anions, or mixed activators.